Recently, because fossil fuel is gradually depleted, the development of various alternative energy resources (i.e. solar cell, fuel cell, wind power) gets more and more attention, particularly the solar power generation.
Referring to FIG. 1, a cross-sectional view of a conventional solar cell element is illustrated, wherein when forming the conventional solar cell element, firstly providing a p-type silicon semiconductor substrate 11 which is etched to form the roughness of surface. Then, a light receiving side of the p-type silicon semiconductor substrate 11 is formed with an n-type diffusion layer 12 of reverse conductive type by heat diffusion using phosphorus or analogues, so as to form a p-n junction. Subsequently, an anti-reflection layer 13 and a front electrode 14 are formed on the n-type diffusion layer 12, wherein a silicon nitride layer is formed on the n-type diffusion layer 12 to be the anti-reflection layer 13 by plasma enhanced chemical vapor deposition (PECVD) thereof. Furthermore, the anti-reflection layer 13 is coated with silver conductive pastes by screen printing, and then processes of curing, drying and high-temperature sintering are carried out to form the front electrode 14. In the process of high-temperature sintering, the silver conductive paste for forming the front electrode 14 can be sintered and penetrate the anti-reflection layer 13 until the silver conductive paste is electrically in contact with the n-type diffusion layer 12.
Furthermore, the back side of the p-type silicon semiconductor substrate 11 uses aluminum conductive paste to form a back electrode layer 15 of aluminum by printing. After, the processes of curing and drying are applied, and the process of high-temperature sintering is carried out, as described above. In the process of high-temperature sintering, the aluminum conductive paste is dried and converted into the back electrode layer 15 of aluminum. Simultaneously, the aluminum atoms are spread into the p-type silicon semiconductor material 11, so that there is a p+ layer 16 having a high concentration of aluminum dopant and formed between the back electrode layer 15 and p-type semiconductor material 11, which is usually called a back surface field (BSF) layer for improving the optical conversion efficiency of solar cell. Because the back electrode layer 15 of aluminum is difficult to weld, the back electrode layer 15 is printed with an aluminum-silver conductive paste thereon by the screen printing, and then sintered to form a conductive wire 17, so that a plurality of solar cells can be serially connected to form a module.
However, the conventional solar cell elements still have the following problems: for example, the front electrode 14, the back electrode layer 15 and the conductive wire 17 are made of silver, aluminum or aluminum-silver conductive pastes. And, the material cost of these conductive pastes is high, and is about 10% of the total cost of the module. Furthermore, the conductive pastes have a predetermined ratio of metal powders, glass powders and organic agent, for example, Japan Kokai Publication No. 2001-127317 and 2004-146521 and Taiwan Pat. No. 1339400 issued to DuPond, wherein the conductive pastes contain glass microparticles that decrease the conductivity and solderability; additionally, the conductive pastes contain the organic solvent therein. Therefore, after sintering, the solar cell chips will be contaminated and thus must be additionally cleaned.
In addition, the electrodes or conductive wires made by the conductive pastes must pass through the high-temperature sintering at 450-850° C. But, at the high temperature, the materials of others material layers may be deteriorated or malfunctioned, and even the yield of manufacturing the cells is seriously affected. Simultaneously, during the sintering of the front electrode 14, the conditions must be controlled, so as to ensure that the conductive pastes of the front electrode 14 completely penetrate through the anti-reflection layer 13 and are electrically in contact with the n-type diffusion layer 12. If the front electrode 14 is not actually in contact with the n-type diffusion layer 12, the manufacture yield of the cells will be dramatically reduced. As described above, according to the precise control requirement of the conditions of the high-temperature sintering, the process of high-temperature sintering needs to consume much time and have more complication, so as to affect the throughput per unit time when generating the cells.
As a result, it is necessary to provide a method for manufacturing electrodes of a solar cell formed by an active solder to solve the problems existing in the conventional technologies, as described above.